Electronic security systems are known for the detection of the unauthorized removal of items from a store, library or other facility. These systems are often employed in retail stores to thwart shoplifting, and in libraries to prevent book theft. Most of the commercially useable security systems are based on the detection of a disturbance in an electromagnetic or magnetic field established in a zone of surveillance. A tag or target in the surveillance zone causes the field disturbance. In those systems which employ a radio frequency electromagnetic field, the tag is usually a resonant circuit sharply tuned to produce an abrupt change in the electromagnetic field when the frequency of the applied field passes through the resonant frequency of the tag. In systems providing a low frequency magnetic field, the tag is typically a high permeability magnetic strip which can abruptly saturate to cause an abrupt change in the magnetic field.
A transmitter and associated antenna provides the electromagnetic or magnetic field in a surveillance zone, and a receiver and associated antenna is operative to detect the presence of the tag as determined by the sensible change in the interrogation field. Upon such detection, an alarm is sounded or other output alarm indication provided.
A preferred electronic security system for the detection of resonant tag circuits is described in U.S. Pat. Nos. 3,810,147; 3,863,244; and 3,967,161. In the system of these patents, an electromagnetic field is provided which is repetitively swept over a predetermined frequency range, the resonant tag having at least one resonant frequency within the swept frequency range. Each time the field passes or sweeps through the resonant frequency of the tag, an abrupt change in the field occurs which is sensed to produce an alarm output.
Magnetic systems are similarly operative and are shown for example in U.S. Pat. Nos. 3,665,449 and 3,747,086.
In electronic security systems such as those described above, two loop antennas are usually employed, one for transmitting and one for receiving. The transmitting loop antenna generates an electromagnetic field which extends far beyond the immediate area of the security system necessary for system operation. In addition, the receiving antenna is sensitive to external noise generated at great distances from the receiver relative to the small area of interest to system operation.
An antenna system is described in U.S. Pat. No. 4,016,553 in which the inherent problems of a simple loop antenna in an electronic security system are minimized by use of two or more identical parallel loop antennas connected in phase opposition or bucking relationship. The antenna system comprises a cluster of at least two parallel electrically conductive loops of similar size connected in phase opposition so that current always flows in mutually opposite directions through corresponding portions of each loop. As a result, the loops are magnetically arranged in a bucking relationship. The length of and spacing between the loops is small compared to the wavelength of the transmitted or received signals and is disclosed to be typically one tenth of the wavelength. The spacing between the parallel loops is an appreciable fraction, for example one fourth, of the width of the egress passage through which a detectable resonant circuit must pass in a security installation. A separate antenna cluster composed of phase opposed parallel loops can be connected to the respective transmitter and receiver of the system, or a single antenna cluster can be employed with both the transmitter and receiver. At distances large compared to the dimensions of the transmitting antenna, the generated electromagnetic waves are cancelled by reason of the phase opposed loop connection. At short distances between the receiving and transmitting antennas, the signals in adjacent parallel antenna conductors do not cancel, resulting in a net detectable signal. Electromagnetic waves incident on the receiving antenna from distances large compared to the antenna dimensions do not provide a sensible antenna signal, but electromagnetic waves from sources close to the antenna are sensed to provide a receiving antenna signal.
Thus the antenna system described in U.S. Pat. No. 4,016,553 provides an electromagnetic field in an interrogation region while preventing high intensity fields from occurring outside of the interrogation region. This antenna system also provides detection of selected electromagnetic fields originating in the interrogation region from a resonant circuit while avoiding detection of fields originating from outside of the interrogation region.
The antenna system described in the aforesaid U.S. Pat. No. 4,016,553 suffers several disadvantages in practice. The bucking loop antennas must be separated by a significant distance relative to the distance between the transmitting antenna cluster and receiving antenna cluster. Moreover, the bucking loop antennas must be carefully aligned and balanced for optimum effect. The loops of an antenna cluster are typically spaced apart from each other by a distance corresponding to one fourth the distance across the egress passage. The size of the antenna cluster can become cumbersome for passage widths of conveniently large dimension. For example, for a passage width of six feet, the antenna cluster must be sufficiently large to accommodate a loop spacing of eighteen inches. An improved antenna system for use with an electronic system for the detection of resonant tag circuits is the subject of U.S. Pat. No. 4,243,980 of the same inventor as herein, and comprises a pair of substantially identical planar multiple loop antennas respectively connected to the transmitter and receiver of the security system and providing an electromagnetic field of high intensity in the interrogation region of the system while preventing high intensity fields at distances outside of the interrogation region which are large in comparison to the antenna dimensions. The antenna system also discriminates against interferring signals originating outside of the interrogation region at distances large compared with the antenna dimensions. Each planar antenna includes two or more loops lying in a common plane, with each loop being twisted 180.degree. with respect to each adjacent loop to be in phase opposition. The transmitting antenna and receiving antenna are symmetrical, that is, identical or nearly so with respect to the number and size of the two or more loops, and are cooperative in that twisted loops of the receiving antenna reverse or decode the adjacent phase relationships of the twisted loops of the transmitting antenna. For each antenna, the total loop area of one phase is equal to the total loop area of the opposite phase in order to minimize the magnetic field generated by the transmitter at distances large compared to the dimensions of the antenna and in order to reject noise at the receiving antenna which is generated at distances large compared to the dimensions of the receiving antenna. The antenna system is also effective to provide higher resonant tag detection sensitivity than conventional loop antennas.
A further improved antenna system is described in U.S. Pat. No. 4,260,990 of the same inventor as herein, in which the two cooperating planar antennas are not symmetrical to each other. The transmitting antenna can be a single loop planar antenna while the receiving antenna can include two or more loops lying in a common plane with each loop being twisted 180.degree. with respect to each adjacent loop. Alternatively, the transmitting antenna can have two planar loops and the receiving antenna three planar loops, the loops of each antenna lying in a common plane with each loop being twisted 180.degree. with respect to each adjacent loop to be in phase opposition.
The antenna systems of the '980 and '990 patents are effective to reject radio frequency interference generated by magnetic fields at distances from the antenna large compared to the antenna dimensions. However, such antennas are still susceptible to electrical noise which is coupled capactively to the antenna.
An antenna system is the subject of U.S. Pat. No. 4,251,808, of the same inventor, which is not sensitive to capacitively coupled noise and which enjoys the advantages of electromagnetic noise rejection as in the systems of the '980 and '990 patents. In the antenna system of U.S. Pat. No. 4,251,808, each multiple loop planar antenna is substantially enclosed within a conductive shield to substantially eliminate capacitive coupling of noise to the antenna. Each multiple loop antenna is enclosed within a metal or other conductive shield which is grounded to provide a shorted turn around the periphery of the multiple loop antenna. The crosswires of the twisted loops are enclosed within a shield portion which is electrically separated from the shorted turn so that no electrical current path is provided through the crossover shield portion. The antenna is totally shielded from capacitive coupling to external sources of noise or spurious signals. If the antenna is perfectly balanced, no currents flow in the shorted turn of the shield since no net voltages are induced into the shield by the antenna magnetic field. If there is a small unbalance in the multiple loop antenna such that a voltage is induced into the shorted turn of the shield, the current flowing in the shield loop tends to cancel out the magnetic unbalance, and thus the shorted shield loop automatically corrects for small unbalances in the multiple loop antenna and reduces the magnetic fields external to the loop at distances outside of the interrogation region.